High Manganese Alloyed Steels With Improved Cracking Resistance

ABSTRACT

The present invention relates to ferrous alloys with high strength, cost-effective corrosion resistance and cracking resistance for refinery service environments, such as amine service under sweet or sour environments. More specifically, the present invention pertains to a type of ferrous manganese alloyed steels for high strength and cracking resistance and methods of making and using the same.

FIELD OF THE INVENTION

This disclosure relates to ferrous manganese-alloyed steels with high strength and good corrosion and cracking resistance for oil, gas, and petrochemical service environments under sweet or sour service.

BACKGROUND OF THE INVENTION

Materials are typically one of the most costly elements in oil and gas exploration and production. Proper selection of materials is becoming increasingly significant to project economics. Carbon steels have conventionally been widely used, and are advantageous because they can be tailored in chemistry and microstructure for strength, toughness, fabricability, and weldability. In addition, carbon steels are relatively low cost structural materials. On the other hand, carbon steels having predominantly ferritic or martensitic phase do not inherently possess cracking or corrosion resistance. Carbon steels with higher strength (e.g., yield strengths higher than 80 ksi) are more susceptible to environmentally-induced cracking such as sulfide-stress-cracking (SSC) or hydrogen-assisted cracking. The American Petroleum Institute (API) has specifications that cover casing and tubing (API specification 5CT) and line pipe (API specification 5-L). Some API line pipe grades can be used for larger diameter casing (e.g., equal to or greater than 16-inch outer diameter).

Many oil and gas fields operate under corrosive conditions in the presence of H₂S, CO₂, and water. The use of corrosion resistant alloys (CRAs) is expanding as the industry is forced to operate in harsher environments, with sourer crude oils and deeper, high pressure and high temperature wells. Austenitic stainless steels are often used and provide a combination of good corrosion resistance, cracking resistance, oxidation resistance, formability, and toughness. These stainless steels owe their good corrosion resistance to high Cr alloying, and their high ductility and toughness to the austenitic crystalline structure stabilized by high Ni alloying. As an example, a commonly used austenitic stainless steel, 304 stainless steel, has a nominal composition of about 18 wt % Cr and 8 wt % Ni. As such, austenitic stainless steel is substantially more expensive than carbon steel, due largely to the high Ni content. Austenitic stainless steels also typically have lower strength than ferritic and martensitic carbon steels and ferritic stainless steels, and are more susceptible to SCC. API specification 5CRA covers corrosion resistant alloys for tubing, casing, and subsurface equipment. Wellhead and christmas tree equipment are covered by API Specification 6A.

SUMMARY OF THE INVENTION

The present invention relates to ferrous alloys with high strength, cost-effective corrosion resistance, and cracking resistance. More specifically, the present invention pertains to a type of ferrous manganese-alloyed steel and methods of making and using the same.

In one aspect, the present invention relates to a ferrous austenitic steel that comprises less than about 30 wt %, 20 wt %, 15 wt %, 10 wt %, 5 wt %, or 3 wt % chromium (Cr) equivalent and at least about 7 wt %, 10 wt %, 12 wt %, 15 wt %, or 20 wt % nickel (Ni) equivalent, wherein

-   -   Ni equivalent is: Ni_(eq)=Ni+Co+0.5·Mn+0.3·Cu+25·N+30·C;     -   Cr equivalent is:         Cr_(eq)=Cr+2·Si+1.5·Mo+5·V+15.5·Al+1.75·Nb+1.5·Ti+0.75·W; and     -   wherein the Ni equivalent and Cr equivalent satisfy         6·Ni_(eq)+Cr_(eq)≥15 and Ni_(eq)+15≥1.5·Cr_(eq), and the ferrous         austenitic steel comprises a predominantly austenite phase         (e.g., at least 80 vol %, 90 vol %, 95 vol %, or 99 vol %         austenite) and optionally one or more minor (e.g., less than 20         vol %, 10 vol %, 5 vol %, 1 vol %, or 0.2 vol %) phases of         ferrite, martensite, carbide, nitride, and carbonitride. In the         formulas for calculating Ni equivalent and Cr equivalent, Ni,         Co, Mn, Cu, etc. refer to the amount of each respective element         that is present in the steel in wt %.

In one aspect, the ferrous austenitic steel comprises at least 8 wt %, 10 wt %, 12 wt %, or 15 wt % and less than 20 wt %, 25 wt %, or 30 wt % manganese (Mn).

In one aspect, the ferrous austenitic steel has a strength level ranging from 20 ksi to 120 ksi or 205 MPa to 900 MPa.

In one aspect, the ferrous austenitic steel comprises at least 0.1, 0.3, or 0.5 wt % to less than 1.5, 1.0, or 0.8 wt % carbon, and at least 0.001 wt % to less than 1.0, 0.8, or 0.5 wt % nitrogen.

In one aspect, the ferrous austenitic steel comprises at least 0.05 wt % to less than 15 wt %, 10 wt %, 5 wt %, 1, or 0.5 wt % Al.

In one aspect, the ferrous austenitic steel comprises at least 0.05 wt % or 0.1 wt % to less than 10 wt %, 5 wt %, 1 wt %, 0.5, or 0.25 wt % Si.

In one aspect, the ferrous austenitic steel of the present invention comprises at least about 0.01 wt %, 0.05 wt %. 0.1 wt %, 0.3 wt %, or 0.5 wt % to less than 3.0 wt %, 2.0 wt %, or 1.0 wt % Cu.

In one aspect, the ferrous austenitic steel of the present invention comprises one or more of niobium (Nb), titanium (Ti), vanadium (V), tungsten (W), tantalum (Ta), and molybdenum (Mo), wherein the total content of these elements ranges from at least 0.01 wt % or 0.3 wt % to less than 5 wt %, 3 wt %, or 2 wt %.

In one aspect, the present invention relates to a process for manufacturing the ferrous austenitic steel disclosed herein, which comprises:

-   -   melting ferrous steel constituents to produce liquid alloy steel         of inventive composition in a controlled environment wherein         evaporation losses of N and Mn are controlled;     -   ingot or continuous casting the liquid alloy steel into mold         (e.g., water-cooled cooper mold for continuous casting) to form         cast ingots while suppressing Mn segregation;     -   reheating the cast ingots to dissolve secondary phases (e.g.,         carbides, nitrides, carbonitrides) at a temperature ranging from         900° C. to 1250° C.;     -   hot deforming at or above 600° C. to control grain size and         shape of alloy steel;     -   cooling rapidly at at least about 10° C./sec to below about 300°         C.

In one aspect, the present invention relates to a process for manufacturing the ferrous austenitic steel disclosed herein, which further comprises improving the mechanical properties of the ferrous austenitic steel properties using a thermo-mechanical controlled processing.

In one aspect, the present invention relates to a petroleum refining or chemical production equipment comprising the ferrous austenitic steel disclosed herein.

Further aspects, features, and advantages of the present invention will be apparent to those of ordinary skill in the art upon examining and reading the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating embodiments of the invention and are not to be construed as limiting the invention. Further objects, features, and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures.

FIG. 1 illustrates a representative Fe—Cr—Mn—Ni phase diagram for ferrous austenitic steels of the present invention.

FIG. 2 illustrates a representative diagram of a sour service environment in which the ferrous austenitic steels of the present invention may be particularly useful.

FIG. 3 illustrates representative NACE TM0177 method A test results of a ferrous austenitic steel of the present invention. Tests were carried out in NACE solution A, with 1 bar H₂S pressure, and ambient temperature with 90% of actual yield strength applied.

FIG. 4 illustrates representative NACE TM0177 method D test results of a ferrous austenitic steel of the present invention. Tests were carried out in NACE solution A, with 1 bar H₂S pressure, and ambient temperature.

FIG. 5 illustrates a representative corrosion rate of a ferrous austenitic steel of the present invention as a function of corrosion inhibitor (CI) dosage. Corrosion tests were carried out under simulated sweet production environment at pH ˜5.2 and ambient temperature.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description provided to aid those skilled in the art in practicing the present invention. Those of ordinary skill in the art may make modifications and variations in the embodiments described herein without departing from the spirit or scope of the present invention. All publications, patent applications, patents, figures and other references mentioned herein are expressly incorporated by reference in their entirety.

The present invention relates to ferrous austenitic steels with high manganese (Mn) alloying, which provides superior sour cracking and hydrogen embrittlement resistance to carbon steels. The ferrous austenitic steels also have relatively high strengths at lower cost, and higher SCC resistance compared to conventional CRAs. The ferrous alloyed steels are predominantly or entirely austenite, with minor phases that may include ferrites, martensites, intermetallic phases, carbides, nitrides, carbonitrides, borides, oxides and combinations thereof. One embodiment of the present invention includes alloyed steel with higher strength (e.g., 60 ksi or higher yield strength) facilitated by significant (e.g., not less than 0.08 wt %) interstitial alloying of carbon (C), nitrogen (N), boron (B), and combinations thereof.

As used herein, “austenite,” also known as gamma-phase iron (γ-Fe), refers to a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727° C.); other alloys of steel have different eutectoid temperatures.

As used herein, “austenitization” refers to heating the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite. The more open structure of the austenite is then able to absorb carbon from the iron-carbides in carbon steel. An incomplete initial austenitization can leave undissolved carbides in the matrix. “Austenitic stainless steel” refers to a specific type of stainless steel alloy. Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic and duplex. These stainless steels possess austenite as their primary crystalline structure (face centered cubic). This austenite crystalline structure is achieved by sufficient additions of the austenite stabilizing elements nickel, manganese and nitrogen. Due to their crystalline structure austenitic steels are not hardenable by heat treatment and are essentially non-magnetic. There are two subgroups of austenitic stainless steel: 300 series stainless steels achieve their austenitic structure primarily by a nickel addition, while 200 series stainless steels substitute manganese and nitrogen for nickel, though there is typically still a small nickel content.

As used herein in the specification and in the claims, “nickel equivalent,” or “Ni_(eq)” refers to calculation for the austenite stabilizing elements. Nickel equivalent has been determined with the most common austenite-stabilizing elements:

Ni_(eq)=Ni+Co+0.5·Mn+0.3·Cu+25·N+30·C.

See, R. Honeycombe, H. K. D. H. Bhadeshia, Steels Microstructure and Properties, 2nd Edition, Edward Arnold, 1995, Page 255.

As used herein in the specification and in the claims, “chromium equivalent,” or “Cr_(eq)” refers to calculation for the ferrite stabilizing elements. Chromium equivalent has been empirically determined using the most common ferrite-stabilizing elements:

Cr_(eq)=Cr+2·Si+1.5·Mo+5·V+15.5·Al+1.75·Nb+1.5·Ti+0.75·W

See, R. Honeycombe, H. K. D. H. Bhadeshia, Steels Microstructure and Properties, 2nd Edition, Edward Arnold, 1995, Page 254.

The steel chemistry can be tailored to use Mn+Ni along with carbon and nitrogen alloying to stabilize the austenite phase (γ-phase) and provide toughness and cracking resistance, while a limited amount of Cr alloying (not more than 30 wt % or 15 wt %) provides suitable corrosion resistance without passivation film formation. Cr is, however, a ferrite phase (α-phase) stabilizer. Hence, as Cr alloying increases, more Mn and/or Ni alloying is required to stabilize the austenite phase. FIG. 1 illustrates a representative Fe—Cr—Mn—Ni phase diagram for ferrous austenitic steels of the present invention.

In an embodiment of the present invention, the alloy chemistry satisfies at least one of the following requirements:

6·Ni_(eq)+Cr_(eq)≥15 and Ni_(eq)+15≥1.5·Cr_(eq),

6·Ni_(eq)+Cr_(eq)≥50 and Ni_(eq)+15≥1.8·Cr_(eq),

6Ni_(eq)+Cr_(eq)≥60 and Ni_(eq)+15≥2.0·Cr_(eq), or

6·Ni_(eq)+Cr_(eq)≥90 and Ni_(eq)+15≥2.5·Cr_(eq).

These steels obtain their predominantly austenitic crystalline structure by replacing the more expensive Ni alloying in conventional austenitic stainless steels with lower cost Mn. Additional strengthening may be achieved via interstitial alloying elements such as C, N, B, and combinations thereof. This reduces costs compared to CRAs (e.g., austenitic 304SS), but still achieves the austenite crystalline structure for high ductility and toughness.

In one embodiment of the invention, these steels may be alloyed with Cr in the range of 0 wt % to 30 wt %, such as from at least 5 wt %, 10 wt %, or 15 wt % to not more than 15 wt % or 30 wt %. This Cr alloying addition provides good corrosion resistance.

The nitrogen solubility in the melt and in the steels may be enhanced by manganese and other alloying elements such as V, Nb, Ti, and Cr, which also stabilize the austenite crystalline structure.

The carbon and nitrogen contents of the inventive steels are selected to provide a range of strength levels ranging from 20 ksi to 100 ksi in as-fabricated condition. The current steels can be cold deformed to achieve even higher yield strength, for instance in excess of 110 ksi.

In an embodiment of the invention, the inventive steels use Cr, Al, and Si alloying, or combination thereof, to promote passivation film formation for corrosion resistance, and use Mn+Ni alloying to stabilize austenite phase (γ-phase) for toughness and cracking resistance.

The high manganese alloyed steels of the present invention are cost-effective materials for sweet and sour service environments. Oil, gas, and petrochemical equipment such as reactor vessels, pipes, casings, packers, couplings, sucker rods, seals, wires, cables, bottom hole assemblies, tubing, valves, compressors, pumps, bearings, extruder barrels, molding dies, and combinations thereof may be made using these steels.

The current invention further includes processes for manufacturing the inventive ferrous austenitic steels, the processes comprising the following steps:

-   -   melting ferrous steel constituents to produce liquid alloy steel         in a controlled environment wherein evaporation losses of N and         Mn are controlled;     -   ingot or continuous casting the liquid alloy steel into a mold         (e.g., a water-cooled cooper mold for continuous casting) while         suppressing Mn segregation;     -   reheating the cast ingots at temperature ranging from 900° C. to         1250° C. to dissolve secondary phases (e.g., carbides, nitrides,         carbonitrides);     -   hot deforming (e.g., rolling, extrusion, piercing, plugging,         forging, pressing) at or above 600° C. to control grain size and         shape the alloy steel into bars, sheets, plates, tubes, or the         like;     -   cooling rapidly at at least about 10° C./sec to a suitable         cooling stop temperature, typically below about 300° C., below         about 200° C., below about 100° C., or to room temperature, to         form predominantly austenitic structure;     -   optionally cold working (e.g., pilgering, drawing, extrusion,         sizing) to enhance the yield strength or adjust the size and         shape of the alloy steel; and     -   optional tempering treatment at or above 150° C. prior to or         after the cold working.

In an embodiment of the invention, the inventive steels can be manufactured by various processing techniques including, but not limited to, thermo-mechanical controlled processing (TMCP). TMCP can produce high strength, low alloy steel plates with refined grain size and microstructure for enhanced mechanical properties. Hot deformation may be applied at or above the recrystallization temperature.

In another embodiment of the present invention, the inventive steels can be strengthened by second phase particles to further improve mechanical strength. For simplicity, the present invention is described primarily in terms of carbide/nitride/carbonitride, but embodiments of the invention could include other precipitates such as borides, oxides, intermetallics.

Carbide in general increases the hardness of materials. The size and spatial distribution of carbide are important. Hard second phase particles increase strength by blocking the dislocation migration during deformation. Fine and uniformly distributed carbide is effective for materials strengthening. Coarse carbide particles can cause mechanical failure of steels.

Carbides may enhance transformation-induced plasticity (TRIP) and/or twinning-induced plasticity (TWIP) effects to improve tensile strength and elongation. The carbon concentration in the carbide phase is much higher than the average concentration for the steel. By mass conservation, carbide largely depletes the carbon in its surrounding matrix. Therefore, TRIP and TWIP could be a predominant deformation mechanism in the carbon depletion zone near carbide precipitates. Compared to fully austenitic steel with the same chemistry, the present invention has higher yield strength and work hardening capability.

To produce fine carbide particles, the carbide should be in a dissolved state before the deformation. Any undissolved carbides will suffer relatively rapid coarsening at elevated temperature. The controlled deformation may take place below the recrystallization stop temperature so that deformation results in elongated austenite grains filled with intragranular crystalline defects, which will be the preferred nucleation sites for carbides. A slow cooling or isothermal holding is then required to promote carbide precipitation. Finally, a rapid quench is applied to keep a fully austenitic matrix. The TMCP method has a synergistic effect of micro-alloy additions. Depending on the alloying elements, proper thermo-mechanical condition should be selected to produce the fine particles.

Alloying elements of the invention have some effect on either the TMCP, the bulk property modification, or both.

Carbon is the most effective alloy element to control the bulk deformation mechanism, promote carbide precipitation, and stabilize the austenite phase during cooling.

Manganese is an austenite phase stabilizer. This element is mainly added to maintain a fully austenitic matrix during cooling and TMCP. It is believed to have little effect on the deformation mechanism.

Chromium is a carbide former and ferrite phase stabilizer. It will promote different types of carbide, such as M₃C and M₂₃C₆, depending on the alloy level and thermal treatment temperature. Chromium addition is important for corrosion resistance enhancement.

Niobium (Nb), Vanadium (V), Titanium (Ti), Molybdenum (Mo) and combinations thereof are effective alloying elements to retard recrystallization during TMCP by forming strain induced carbonitride (e.g., (Ti, Nb) (C,N)) precipitation on the deformed austenite. In addition, these alloying elements help bulk carbon concentration modification.

Aluminum and silicon may be added to tune the stacking fault energy (SFE) of high manganese steel.

In an embodiment of the present invention, ultra-fine grained (≤20 lam or ≤10 lam grain size) high Mn steels can be fabricated by a thermo-mechanical process consisting of heavy plastic deformation at ambient, cryogenic (e.g., liquid nitrogen temperature), or intermediate temperature (e.g., 150° C.-600° C.) to induce martensite (e.g., α′-martensite and/or ε-martensite) formation and subsequent annealing at elevated temperatures to reverse deformation-induced martensite into ultra-fine grained austenite.

Metastable austenite phase can be transformed to strain-induced martensite phase by heavy plastic deformation at ambient or intermediate temperatures. The strain-induced martensite can be further heavily deformed to destroy lath structure prior to reversion treatment. The strain-induced martensite should be reverted to austenite at temperatures low enough to suppress the grain coarsening of the reverted austenite. The chemistry of high Mn steels can be tailored so that the martensite start temperature (Ms), or MD temperature of reverted austenite, is below room temperature.

EXAMPLES Example 1

Steel plates having the nominal chemistry shown in Table 1 were fabricated by vacuum induction melting and hot rolling. Steel plates with 25 mm thickness were fabricated by finish rolling at around 850° C. followed by accelerated cooling (at least 10° C./sec) to room temperature.

TABLE 1 Chemistry (weight percent) and mechanical properties. Yield Yield Strength Strength after cold working Sample C Mn Cr Si Cu Al as-rolled (ksi) ID (%) (%) (%) (%) (%) (%) (ksi) e = 10% e = 20% e = 30% Steel 1 0.6 18 — — — 62.8 — — — Steel 2 0.8 18 — — — 64.8 — — — Steel 3 0.6 18 — — 0.5 65.7 — — — Steel 4 0.6 18 — — 2 68.9 — — — Steel 5 0.6 18 5.2 0.1 — 83.9 — — — Steel 6 0.6 18 9.7 — — 90.1 — — — Steel 7 0.6 18 5.0 3.0 — 86.0 — — — Steel 8 0.59 18.4 5.0 0.1 1.5 56.3 Steel 9 0.59 15.1 5.0 0.1 1.5 59.5 Steel 10 0.59 15.2 0.1 1.5 53.4 Steel 11 0.6 18 3 — — 83.3 112.5 116.3 142.3 Steel 12 0.6 18 — 1 — 67.2 102.0 115.9 130.1

The sour cracking resistance of a selected chemistry of inventive steels was evaluated per NACE TM0177 Method A and Method D testing. Round tensile test specimens of 0.357″ outer diameter were machined from steel samples 11 and Steel 12. Tensile pre-straining of 10%, 20%, and 30% was applied to enhance the yield strength of the steel. The inventive steels exhibited superior cracking resistance (i.e., no cracks observed) as shown in FIGS. 3 and 4 .

Example 2

Alloys were prepared by vacuum induction melting with nominal chemistry of (weight percent) 0.6% C-I 8% Mn-0.1% Si (HMS-0Cr) and 0.6% C-18% Mn-5% Cr-0.1% Si (HMS-5Cr). The evaporation losses of Mn, and Cr were controlled during the vacuum induction melting. The liquid alloyed steel was poured into water-cooled copper molds to form ingots. The cast ingots were then reheated at 1200° C. for 2 hours and hot deformed at temperatures above 800° C. to control grain size and shape the alloy steel into plates, followed by rapid cooling by water quench. Predominantly austenitic structure was achieved with minor secondary phases. The compatibility of these steels with corrosion inhibitor (CI) was investigated, with commercially available organic sulfur compound-based CI (Nalco EC1625A). The solution was 3 wt % NaCl and de-aerated by flowing nitrogen (>12 hrs). Then, the solution was saturated with 1 bar CO₂ gas flow (>6 hrs) having pH 4.5-5.0 to simulate an oil/gas production environment. Corrosion rate was measured by Linear Polarization Resistance (LPR) method upon achieving steady-state after adjusting CI content. The steels, HMS-0Cr and HMS-5Cr, showed significant lower corrosion rate below 4 mpy (mil per year) with 10 ppm or more of CI, which is comparable to that of carbon steel, as shown in FIG. 5 .

Example 3

Alloys were prepared by vacuum induction melting with a chemistry of 0.60% C-18.6% Mn-0.11% Si-0.01N. The evaporation losses of N, Mn, and Cr were controlled during the vacuum induction melting. The liquid alloyed steel was poured into water-cooled Cu (copper) mold to form ingots. The cast ingots were then reheated at 1200° C. for 2 hours and hot deformed at temperatures above 800° C. to control grain size and shape of alloy steel into plates followed by rapid cooling by water quenching. Predominantly austenitic structure was achieved with minor secondary phases. The corrosion rate of the inventive steels was investigated in a 10 wt % NaCl aqueous solution at ambient pressure and temperature. The solution was saturated with 1 bar CO₂ gas flow (>6 hrs) having pH 4.0-5.0 to simulate an oil/gas production environment. Corrosion rate was measured by Linear Polarization Resistance (LPR) method upon achieving steady-state. The inventive steels showed lower corrosion rate below 30 vmpy (mil per year) than that of carbon steel 960 mpy).

Example 4

Steel plates having the chemistry shown in Table 2 were fabricated by vacuum induction melting and hot rolling. Steel plates with 15-25 mm thickness were fabricated by finish rolling at around 800-850° C. followed by accelerated cooling to room temperature. The liquid alloyed steel was poured into water-cooled Cu (copper) mold to form ingots. The cast ingots were then reheated at 1200-1250° C. for 2 hours and hot deformed at temperatures at or above 600° C. to control grain size and shape of alloy steel into plates followed by rapid cooling by water quenching. Predominantly austenitic structure was achieved with minor (less than or equal to 5 vol. % total) secondary phases of carbides, nitrides, and carbo-nitrides, ferrite, and ε-/α-martensite.

TABLE 2 Chemistry (weight percent) and mechanical properties. Sample Chemical composition (wt %) YS UTS El. ID C Mn Cr Si Mo Ti Nb Cu V N (ksi) (ksi) (ksi) Steel 13 0.61 17.5 3.1 0.15 — — — 0.5 — 0.001 86.6 157.4 57 Steel 14 0.60 18.3 3.0 0.13 — — 0.02 0.5 — 0.001 88.6 156.2 55 Steel 15 0.61 18.0 3.0 0.14 — 0.021 0.02 0.5 — 0.014 89.6 157.5 61 Steel 16 0.62 18.1 3.0 0.15 — 0.017 0.02 0.5 — 0.021 91.4 158.1 55 Steel 17 0.60 18.2 3.0 0.12 — — 0.02 0.5 0.1 0.001 90.6 156.8 54 Steel 18 0.59 18.0 3.0 0.16 0.52 0.52 0.02 0.5 — 0.001 90.5 158.6 54 YS: Yield strength, UTS: Ultimate Tensile Strength, El.: Elongation

Example 5

Steel plates having the chemistry shown in Table 3 were fabricated by vacuum induction melting and hot forging. The liquid alloyed steel was poured into water-cooled Cu (copper) mold to form ingots. The cast ingots were then reheated at 1200-1250° C. for 3 hours. Steel plates with 15-25 mm thickness were fabricated by hot forging at around 900° C. followed by rapid cooling by water quenching. The plates were cold rolled at ambient temperature to enhance the yield strength to 100 ksi to 135 ksi range as shown in Table 3. The plates were machined into compact tension (CT) specimens with notches per ASTM E647. The CT specimens were then exposed to high temperature (300° C.), high pressure (2,000 psig) hydrogen for 14 days in a charging chamber. The ingressed hydrogen content was evaluated to be over 26 ppm in steel 19, and 36 ppm in steel 20. J-integral (JO fracture toughness was evaluated on the hydrogen charged CT samples per ASTM E1820. Inventive steels have exhibited over 970 lbf/in fracture toughness which is significantly higher than that of hydrogen charged C110 grade carbon steels (e.g., about 35 lbf/in).

TABLE 3 Chemistry (weight percent) and mechanical properties. Forged Cold-rolled Sample Chemical composition (wt %) YS UTS El. YS UTS El. H2 J_(IC) ID C Mn Cr Si N (ksi) (ksi) (%) (ksi) (ksi) (%) cont. (lbf/in) Steel 19 0.57 17.7 0.01 0.02 0.001 42.8 139.8 71.0 134.0 182.8 28.9 26 975, 1225 Steel 20 0.58 17.9 4.99 0.02 0.002 48.9 131.7 89.3 122.9 164.2 39.4 36 1175, 1215 YS: Yield strength, UTS: Ultimate Tensile Strength, El.: Elongation

Example 6

In another exemplary embodiment of the disclosure, a steel plates having the chemistry shown in Table 4 was fabricated by vacuum induction melting and hot forging. The liquid alloyed steel was poured into water-cooled Cu (Copper) mold to form ingots. The cast ingots were then reheated at 1200-1250° C. for 3 hours. Steel plates with 15-25 mm thickness were fabricated by hot forging at around 900° C. followed by rapid cooling by water quenching. The plates were cold rolled at ambient temperature to enhance the yield strength to over 100 ksi range as shown in Table 4. The plate was machined into double cantilever beam (DCB) specimens with notches per NACE TM0177 method D. The DCB specimens were then exposed to high temperature (300° C.), high pressure (20,000 psig) for 19 days. Over 100 ppm of hydrogen was ingressed into the steel. The liner elastic fracture toughness (Kw) was evaluated to be 36 ksi-gin.

TABLE 4 Chemistry (weight percent) and mechanical properties. Forged Cold-rolled Sample Chemical composition (wt %) YS UTS El. YS UTS El. H2 K_(IC) ID C Mn Cr Si N (ksi) (ksi) (%) (ksi) (ksi) (%) cont. (ksi√in) Steel 21 0.49 17.9 3.01 0.03 0.002 43.5 129.4 75.1 103.5 154.5 36.7 106 36 YS: Yield strength, UTS: Ultimate Tensile Strength, El.: Elongation 

1. A ferrous austenitic steel comprising less than 15 wt % chromium (Cr) equivalent and more than 7 wt % nickel (Ni) equivalent, wherein Ni equivalent is: Ni_(eq)=Ni+Co+0.5·Mn+0.3·Cu+25·N+30·C; Cr equivalent is: Cr_(eq)=Cr+2·Si+1.5·Mo+5·V+15.5·Al+1.75·Nb+1.5·Ti+0.75·W; and wherein the Ni equivalent satisfies 6·Ni_(eq)+Cr_(eq)≥15, and the Cr equivalent satisfies Ni_(eq)+15≥1.5·Cr_(eq); and the ferrous austenitic steel comprises a predominantly austenite phase and one or more minor phases of ferrite, martensite, carbide, nitride, and carbonitride.
 2. The ferrous austenitic steel of claim 1, further comprising 18 wt % to 30 wt % manganese (Mn).
 3. The ferrous austenitic steel of claim 1, wherein the ferrous austenitic steel has a strength ranging from 20 ksi to 120 ksi.
 4. The ferrous austenitic steel of claim 1, further comprising 0.01 wt % to 2 wt % copper (Cu).
 5. The ferrous austenitic steel of claim 1, further comprising 0.1 wt % to 1.5 wt % carbon and 0.001 wt % to 1.0 wt % nitrogen.
 6. The ferrous austenitic steel of claim 1, further comprising 0.05 wt % to 15 wt % Al.
 7. The ferrous austenitic steel of claim 1, further comprising 0.05 wt % to 10 wt % Si.
 8. The ferrous austenitic steel of claim 1, further comprising one or more of niobium (Nb), titanium (Ti), vanadium (V), tungsten (W), tantalum (Ta), and molybdenum (Mo), wherein the total content of these elements ranges from 0.01 wt % to 5 wt %.
 9. The ferrous austenitic steel of claim 1, further comprising 0.1 wt % to 1.5 wt % carbon, 0.001 wt % to 1.0 wt % nitrogen, 0.05 wt % to 10 wt % Al, 0.1 wt % to 3 wt % Si, and 0.01 wt % to 5 wt % of one or more of niobium (Nb), titanium (Ti), vanadium (V), tungsten (W), tantalum (Ta), and molybdenum (Mo).
 10. The ferrous austenitic steel of claim 1, further comprising 0 wt % to 5 wt % Cr equivalent.
 11. The ferrous austenitic steel of claim 1, wherein the austenite phase is at least 95 vol %.
 12. The ferrous austenitic steel of claim, wherein the minor phases are less than 5 vol %.
 13. A processes for manufacturing the ferrous austenitic steel according to claim 1, the process comprising: melting ferrous steel constituents while controlling evaporation losses of N and Mn to produce a liquid alloy steel having the composition of claim 1; ingot or continuous casting the liquid alloy steel into a mold to form cast ingots while suppressing Mn segregation; reheating the cast ingots to dissolve secondary phases at a temperature ranging from 900° C. to 1250° C.; hot deforming at or above 600° C. to control grain size and shape of the alloy steel; and cooling rapidly at at least about 10° C./sec to below about 300° C.
 14. The process for manufacturing the ferrous austenitic steel according to claim 13, further comprising improving the mechanical properties of the ferrous austenitic steel using a thermo-mechanical controlled processing. 